In my family we are using a similar approach to noramum. We started by reading everything in the minority language (Spanish). We had some books in Spanish, but a lot of them were also in English because there wasn't much choice at the library. I found it easy to read them as I could translate them into Spanish, and so did my husband (who also speaks Spanish at home even though he is a native English speaker).
When my son was 2, we found out he wasn't saying much at his nursery and I sort of worried that he was struggling to communicate due to not having enough vocabulary to express himself in English as opposed to what he could do in Spanish. I suggested to my husband to start reading DS one of the bedtime stories in English, and my son at first completely rejected this saying that his daddy was speaking the wrong language. Every time we gave him a choice of having the book read in Spanish or English he always chose Spanish. We went along with it. Eventually he started accepting the other language as well. Since he wouldn't let us read him in English, I introduced the vocabulary he needed by reading his bilingual picture dictionary and saying "we said this in Spanish, how do other people call this in English?" And then I saw a lot of improvement in his acceptance of the other language and not long after that we heard he started talking a lot in English at the nursery.
Going back to the bedtime story, as the books get longer and become more difficult to translate on the spot, I either tell him an oral version of the story in my language looking at the pictures or tell him that the book is in English and he needs to ask his dad to read it in English, I read to him the books in Spanish. My son is almost 3 now, so I don't have any experience regarding books from school, homework, GCSE, yet.
I didn't attempt to translate the Gruffalo, because as everyone else said, it worked so beautifully in English that I felt it lost too much in translation if I tried to read it to him in Spanish. That is, until I found out there was a copy of "El Gruffalo" and I got it for him. This version is wonderful as well, and I love reading it to him. I quite like having both versions of the same book and by reading them in different languages comparing the way in which they are expressed.
But I know that once we get to the long books with chapters, it would probably be a good idea to read the books in their original language. We'll see. It's a fascinating topic anyway.
Incidentally we've also got books in Italian from the library (I've always liked the sound of that language), and one in French that my husband can read to him. DS has since discovered there are many other languages apart from the two that he knows and keeps askings us how to say various things in other languages.